The invention relates to an apparatus for use in magnetoelectric crack detection, such as an apparatus comprising a sensor in the form of a Hall generator or magnetoresistor disposed in a retaining device and generating without contact a measuring quantity corresponding to the current in a test current conductor. The conductor may be a separate test conductor or may be the test-piece itself when appropriate. The apparatus may be used in the magnetic particle process for detecting cracks on the surfaces of work-pieces.
Magnetoelectric crack-detection is an important non-destructive test method. In the method, the test-pieces have to be magnetized so that the surface to be detected can be indicated by the resulting magnetic leakage flux. These magnetization processes are classified in DIN 54130. Except for magnetization by permanent magnets, which is economically unimportant. magnetization is always brought about by a predetermined current which generates a field in a coil which is then conveyed through the test piece, or alternatively the field-producing current is conveyed directly through the work-pieces (test-piece). The value of the current defines the value of the tangential field strength, which is critical in this process. This connection is particularly clear in the case of direct automatic flux (code letters SS in DlN 54130).
However, the current value is dependent not only on the characteristic of the qenerator but particularly on the total resistance of the the circuit. Particularly large inaccuracies are caused by fluctuations in contact resistance resulting from inaccurate coupling of the test-piece to the test circuit, or through scale or similar influences.
The current which actually flows, therefore, is one of the most important test preconditions for the entire process. If it is within the set tolerance range and can be kept constant, by a manual readjustment or by an electronic control system, the most important test precondition is fulfilled.
Owing to the importance of the correct current flow in magnetoelectric crack testing, it has long been the practice to measure the test current directly in high quality apparatus. The test current is characterized by high values (between 10 A and 20,000 A) and low voltages (below 42 V). It is known to measure these currents by using shunt resistors (DIN 43703) in the case of current generator for magnetoelectric crack testing. In shunt resistors the voltage drop in calibrated conductors is the quantity for measuring the current flowing. Shunt resistors are suitable for DC and AC but have the disadvantage that they cannot indicate rms values for the currents, which are mainly phase-controlled. This disadvantage is particularly serious in the case of phase-controlled AC and phase controlled half-wave DC, since DIN 54131 part 1 specifically describes the rms value as the control value for magnoelectric fault testing. Another disadvantage of shunt resistors is that a special size is needed for each current range. For example, about 35 sizes are needed for the range from 1 A to 15,000A. Each size is large and heavy. For example a shunt resistance for 1O,OOOA measures about 185 .times.206 .times. 170. Another disadvantage is the high price. As a result of these shunt resistors, the generators are large, heavy and expensive. Storage is also difficult and expensive, owing to the wide variety of parts. It becomes correspondingly difficult to supply spare parts on a worldwide scale.
It is also known to use current transformers to measure the test current in the case of magnetoelectric crack detection. In this case the high current is conveyed to a coil. The secondary current produced in the coil is proportional to the test current and therefore serves as a value for measuring. However, current transformers are suitable only for AC. The rms value of the current, which is required for detecting surface cracks, can be determined only approximately. using a moving-iron instrument. As before, each size is suitable only for a limited range of current, and there are over thirty sizes in the current range of 1OA to 20,000A used for magnetoelectric crack detection. The space needed for current transformers is even greater than for shunt resistors, and the prices are likewise high. However, the most serious disadvantage both of current transformers and of shunt resistors is that neither system is universal but is suited for special kinds of currents. According to DIN 54130, however, all kinds of currents, e.g. DC, AC, half-wave DC and full-wave DC, are used depending on the purpose of the test, the shape of the test-piece and the required depth of penetration. Generators for magnetoelectric crack detection are therefore often constructed so that they can supply a number of kinds of current. For example an AC/DC generator can supply DC, AC and half-wave DC to DIN 54130. In order to determine the prescribed rms value of the phase-controlled current in this case, therefore, these devices must contain a DIN 43703 shunt resistor for DC and also a transformer and moving-iron instrument for AC.
It is known to use a Hall sensor or "Lohet" for vehicle ignition distributors or in sewing-machine needle control systems for controlling the travel or angle of rotation or amplitude of oscillation. Such devices serve as a current or magnetic barrier or for position control or the like, usually with an additional permanent magnet. Furthermore, in a particular power current system a Hall generator is rigidly incorporated in a resin in a pot-shaped magnet core of compacted powder, to give better external inductive screening to the sensor.
The details of these arrangements are closely related to a particular construction irrelevant to fault- detecting devices, or to general physical properties of the Hall sensor.